This invention relates to the preparation of synthetic organic polymers. More particularly, this invention relates to the preparation of polyacetals and polyketals in aqueous media containing emulsified aldehydes or ketones and polyfunctional alcohols.
Polyacetals and polyketals are particularly preferred classes of condensation polymers because their properties make them suitable for a variety of end-use applications including films, coatings and engineering plastics. By judiciously selecting monomers and polymerization conditions, it it possible to optimize those properties desired for a particular end use.
A conventional method for preparing polyacetals and polyketals is by the reaction of the appropriate mono- or difunctional carbonyl compound with a polyfunctional alcohol. It has been reported by Hill et al [J.A.C.S. 45, 3124 (1923)] that the acid catalyzed reaction of acetaldehyde with diols containing up to 6 carbon atoms yields cyclic acetals. The reaction products of acetaldehyde with octamethylene glycol and decamethylene glycol are reportedly viscous, non-distillable syrups which were presumed to be linear polymers. In all instances the reactants were heated in the presence of a catalytic amount of sulfuric acid without employing any diluents. The authors report that the polyacetals were readily hydrolyzed in the presence of dilute mineral acids to yield the corresponding aldehyde and difunctional alcohol.
U.S. Pat. No. 2,071,252 to Carothers discloses the production of linear polyacetals by the reaction of certain difunctional alcohols with acetals derived from relatively low molecular weight monofunctional alcohols and a variety of aldehydes. The reactions were conducted at the boiling point of the reaction mixture and the low molecular weight alcohol produced as a by-product of the reaction was continuously removed by distillation. This reference also teaches that in some instances the initial acetal can be prepared in situ by heating a mixture containing the desired aldehyde, monofunctional and difunctional alcohols and a suitable catalyst. Some of the polyacetals, for example the one derived from 1,10-decanediol and formaldehyde, are solid materials exhibiting melting points below 100.degree. C. These materials were further polymerized in a molecular still to yield a residue of a fiber-forming material and up to 50% by weight of a distillate corresponding to a cyclic dimer, which reportedly resulted from some decomposition of the initial reaction mixture. This method is less than desirable since it requires using a monofunctional alcohol that must subsequently be distilled from the reaction mixture. In addition, distillation of the alcohol requires that the reaction mixture be heated to the boiling point, which necessitates a considerable expenditure of energy. The products of the final polymerization reaction are viscous liquids or solids, both of which would be difficult to remove from the reaction vessel and purify, particularly if commercial scale quantities of reactants were used.
U.S. Pat. No. 2,963,464 to Cohen et al. discloses the reaction of pentaerythritol and/or dipentaerythritol with difunctional aldehydes to form polyspiranes. The reaction is conducted in the presence of an acidic catalyst and an inert liquid that is a solvent for at least one of the reactants. One example in this patent describes the product obtained by reacting pentaerythritol and glutaraldehyde in boiling water using dodecylbenzenesulfonic acid as the catalyst. The product is reportedly a solid which did not melt below 300.degree. C., was soluble in a mixture of isomeric cresols and insoluble in water and all of the common non-phenolic organic solvents tested. Such a product would be of relatively little commercial value, since it could only be processed as a solution in a phenolic solvent. Phenols and certain derivatives thereof are undesirable solvents due to their odor, volatility, toxicity and relatively poor oxidative stability. Present legislation limiting environmental pollution to specified levels would make it very difficult to conduct a commercial scale process for transforming bulk polymer into shaped articles or incorporating the polymer into a coating formulation using phenol or the isomeric cresoles as a solvent.
One objective of the present invention is to provide a commercially feasible method for preparing polyacetals and polyketals of useful molecular weight.
A second objective of this invention is to provide polyacetal and polyketal compositions that can readily be employed for a variety of end uses while avoiding the potential environmental pollution problems associated with the use of organic solvents.
It is known that the reaction between a monofunctional alcohol and a carbonyl compound to form an acetal or ketal involves an equilibrium that can be represented by the equation. EQU 2 ROH+R'R"C.dbd.O.revreaction.R'R"C(OR).sub.2 +H.sub.2 O.
In the foregoing equation R and R' are hydrocarbyl groups and R" is hydrogen or a hydrocarbyl group. The prior art teaches that removal of at least one of the two products from the liquid phase of the reaction mixture is essential to avoid hydrolysis of the acetal or ketal, particularly in acidic media. This requirement also applies to the production of polymeric compounds, the only difference being that the alcohol represented by ROH in the foregoing equation is polyfunctional.
A disadvantage associated with preparing polyacetals and polyketals by conventional bulk polymerization techniques is that the high viscosity exhibited by products which are liquid or semi-solid at ambient temperature would make these products difficult to transfer and process. Solid products could require mechanical processing to break up large pieces in order to remove them from the reactor.
One method for avoiding the problems associated with the manufacture and processing of relatively high molecular weight polymers in molten or solubilized form is to employ a technique known as emulsion polymerization whereby one or more monomers are reacted in an aqueous medium containing a catalyst and, usually, a surfactant. The final polymer is obtained as an aqueous emulsion or latex exhibiting a relatively low viscosity which sometimes approaches that of water. Heretofore emulsion polymerization employing water as the continuous phase has been employed substantially exclusively for the polymerization of ethylenically unsaturated compounds in the presence of free radical sources, such as organic perixodes. Since polyacetals and polyketals are susceptible to hydrolysis, particularly in the presence of acidic catalysts, emulsion polymerization in aqueous media has heretofore not been considered a practical means for preparing polyacetals and polyketals in general, and particularly those derived from monofunctional aldehydes or ketones.
It is known to prepare certain types of condensation polymers, particularly polyamides, by interfacial polymerization. In accordance with this method, an aqueous phase containing a solubilized or emulsified diamine such as hexamethylene diamine, usually in the form of the corresponding sodium salt, is combined with a water-immiscible organic liquid such as methylene chloride containing a solubilized diacyl halide such as sebacoyl dichloride. At relatively low temperatures a rapid, and in some cases instantaneous, formation of solid polymer occurs at the interface between the two liquid layers. If the reaction is to proceed to completion, the polymer must be continuously removed from the area of the interface by stirring the reaction mixture or by withdrawing the polymer from the interfacial region as the reaction progresses.
Interfacial polymerization has not heretofore been employed for the production of polyacetals and polyketals because the rate of reaction of polyfunctional alcohols with carbonyl compounds is considerably slower than the rate of reaction of alcohols or amines with acyl halides. The acyl halides react so rapidly that no catalyst is required. By comparison, the reaction of carbonyl compounds with polyhydric alcohols requires a catalyst to achieve a useful reaction rate.
Since both interfacial polymerization, as it has been applied to the formation of polyamides, and emulsion polymerization employ an aqueous phase, this relatively large amount of water could be expected to displace the equilibrium of the aldehyde-alcohol reaction in the direction of hydrolysis of any soluble polyacetal formed to the corresponding polyfunctional aldehyde and alcohol, particularly in the presence of acidic catalysts. It is therefore not obvious to employ either of these techniques as a means for preparing commercially useful polyacetals that are even slightly soluble in the reaction medium.
Surprisingly it has now been found that polyacetals and polyketals wherein the average number of repeating units per molecule is as high as 20 or more can be prepared by emulsion polymerization in aqueous media and in the presence of specified polycondensation catalysts.